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    PWM Signal with PIC 16F84

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    thaers3d

    عدد المساهمات : 23
    تاريخ التسجيل : 2009-03-12

    PWM Signal with PIC 16F84

    Post  thaers3d on Thu Apr 30, 2009 1:45 am

    [left][PWM Signal with PIC 16F84
    Creating A PWM Signal Using A PIC 16F84
    There are many small mechanisms, particularly servo motors, that use PWM coding as a
    means of input. PWM signals can also be used to vary the voltage applied to a device by
    achieving an effective average voltage. With so many applications, it is therefore necessary to
    have a reliable means of generating a PWM signal.
    MOTIVATION AND AUDIENCE
    The focus of this tutorial is to demonstrate a method of generating a PWM signal using a PIC
    16F84. This tutorial will teach you:
    l What a PWM signal is.
    l How to write code to generate a PWM signal using a PIC 16F84.
    To do this, it is assumed that you already:
    l Have completed "A Fast Track to PIC Programming".
    The rest of the tutorial is presented as follows:
    l Parts List and Sources
    l Background
    l Programming
    l Applications
    l Final Words
    PARTS LIST AND SOURCES
    In order to complete this tutorial you must have the circuit from the tutorial "A Fast Track to
    PIC Programming" (minus the dip switches and resistor LED circuits). This circuit will be the
    only part required for this tutorial. You will also need a DC power supply and access to an
    oscilloscope to observe the signal.
    BACKGROUND

    PWM Signal with PIC 16F84
    Figure 1
    A PWM signal is simply a pulse of varying length, in effect a rectangular wave. This is illustrated
    in Figure 1, which also shows how a servo might react to different PWM inputs. For our circuit,
    the maximum voltage outputted will be +5 VDC, and the minimum will be 0 VDC. The length of
    the pulse generated is some times charcterized by a duty cycle. The duty cycle is the
    percentage of the signal that the output remains high. For instance, a constant +5V would be
    equivalent to a 100% duty cycle. A typical square wave output from a function generator has a
    50% duty cycle. 0V would correspond to a 0% duty cycle.
    PROGRAMMING
    PWM.asm
    ; FILE: PWM.asm
    ; AUTH: Keith Sevcik
    ; DATE: 5/21/03
    ; DESC: This program generates a PWM waveform.
    ; NOTE: Tested on PIC16F84-04/P
    ;----------------------------------------------------------------------
    ; cpu equates (memory map)
    list p=16f84
    radix hex
    ;----------------------------------------------------------------------

    PWM Signal with PIC 16F84
    portb equ 0x06 ; port b equate
    duty equ 0x0c ; length of duty cycle
    temp equ 0x0d ; length of duty cycle
    ;---------------------------------------------------------------------
    c equ 0 ; status bit to check after subtraction
    ;---------------------------------------------------------------------
    org 0x000
    movlw 0x00 ; load W with 0x00 make port B output
    tris portb ; copy W tristate to port B outputs
    movlw 0x00 ; fill w with zeroes
    movwf portb ; set port b outputs to low
    rstrt movlw d'0'
    movwf portb
    movlw d'157' ; Duty cycle length
    movwf duty
    b0loop movf duty,w
    movwf temp
    bsf portb,0
    pwma nop
    nop
    nop
    nop
    nop
    nop
    nop
    nop
    nop
    nop
    nop
    nop
    decfsz temp
    goto pwma
    movlw d'255'
    movwf temp
    movf duty,w
    subwf temp,f
    bcf portb,0
    pwmb nop

    PWM Signal with PIC 16F84
    nop
    nop
    nop
    nop
    nop
    nop
    nop
    nop
    nop
    nop
    nop
    decfsz temp
    goto pwmb
    goto rstrt
    ;----------------------------------------------------------------------
    end
    ;----------------------------------------------------------------------
    ; at burn time, select:
    ; memory uprotected
    ; watchdog timer disabled
    ; standard crystal (4 MHz)
    ; power-up timer on
    HEADER AND EQUATES
    The first portion of code is the header and register equates. For more information about the
    meaning of the header see the previous tutorial.
    list p=16f84
    radix hex
    ;----------------------------------------------------------------------
    portb equ 0x06 ; port b equate
    duty equ 0x0c ; length of duty cycle
    temp equ 0x0d ; length of duty cycle
    ;---------------------------------------------------------------------
    c equ 0 ; status bit to check after subtraction

    PWM Signal with PIC 16F84
    ;---------------------------------------------------------------------
    org 0x000
    The only equate of signifficance here is PWM. This register will be used to store the length of
    the PWM signal to be generated.
    INSTRUCTIONS
    The next portion of code contains the actual instructions that tell the PIC what to do.
    start movlw 0x00 ; load W with 0x00 make port B output
    tris portb ; copy W tristate to port B outputs
    movlw 0x00 ; fill w with zeroes
    movwf portb ; set port b outputs to low
    These lines set up port B as outputs. All outputs are then set to low.
    rstrt movlw d'0'
    movwf portb
    movlw d'157' ; Duty cycle length
    movwf duty
    After setting up the ports, the main loop is begun. At the beginning of the main loop, all port b
    pins are set to low just incase they are high when they shouldn't be. The duty cycle is then set
    to 157 (a 50% duty cycle. 255 corresponds to 100% and 0 corresponds to 0%).
    b0loop movf duty,w
    movwf temp
    bsf portb,0
    pwma nop
    nop
    nop
    nop
    nop
    nop
    nop
    nop
    nop
    nop

    PWM Signal with PIC 16F84
    nop
    decfsz temp
    goto pwma
    The next bit of code is the loop for the PWM signal generated at pin B0. The pwm1a loop
    generates the high portion of the PWM signal. The duty cycle is stored in temp and then the pin
    is set high. after a pause, temp is decremented and so long as it doesnt reach zero the pause is
    repeated and temp is decremented again. After temp reaches zero, the code continues.
    movlw d'255'
    movwf temp
    movf duty,w
    subwf temp,f
    bcf portb,0
    pwmb nop
    nop
    nop
    nop
    nop
    nop
    nop
    nop
    nop
    nop
    nop
    nop
    decfsz temp
    goto pwmb
    goto rstrt
    The next portion of code generates the low part of the PWM signal. The value 255 is stored in
    temp, and the duty cycle is subtracted from this. This gives the remaining length of signal to be
    generated. Temp is then decremented in the same manner as above, this time with B0 set to
    low. Once the entire PWM signal has been generated, the code repeats.
    This code causes a PWM signal to be generated with a duty cycle proportional to the value set.
    The frequency of the signal can also be adjusted by varying the delay (the number of nop's
    used).
    APPLICATIONS
    One common application of pwm signals is motor control. By varying the duty cycle of a pwm
    signal sent to a motor, you can vary the effective power of the signal and thereby slow the
    http://www.pages.drexel.edu/~kws23/tutorials/PWM/PWM.html (6 of 8)14.03.2006 16:51:58
    PWM Signal with PIC 16F84
    motor down or speed the motor up depending on how long of a pulse you send to the motor.
    The signal generated by the PIC can not be directly connected to the motor, however, because
    the PIC is unable to handle the power required by the motor. It is therefore necessary to use a
    transistor to regulate the flow of current to the motor. A transistor is like an electric switch.
    When you send a logic high (+5V) to the transistor, it allows current to flow. When a logic low
    (0V) is sent, it restricts the flow of current. For digital signals, this means that the signal can be
    reproduced exactly, except the new signal is scaled up to a much larger current. Figure 2
    shows a schematic for controlling a motor using a TIP31 NPN transistor.
    Figure 2
    As the schematic shows, the output from the pick is wired to the base. The negative terminal of
    the motor is then connected to the base and the collector is connected to ground. When the
    PWM otuput from the PIC is sent to the transistor, it will flip the transistor on and off and
    subsequently generate the same PWM signal to the motor, allowing you to control the motor
    with a PWM signal.


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